Document supported pseudocode with section 7 language manuals

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

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+% CPSC-KEYWORD-FUNCTION(7)
+
+# NAME
+
+cpsc-keyword-function - define a function that returns a value
+
+# SYNOPSIS
+
+```text
+FUNCTION name(parameters) RETURNS type
+    statements
+    RETURN expression
+ENDFUNCTION
+```
+
+# DESCRIPTION
+
+Call a function in an expression as `name(arguments)`. A call may precede its
+top-level definition. RETURNS specifies the result type; omitting it defaults
+to INTEGER. RETURN supplies a value of that type. Whole-array results are
+unsupported.
+
+Parameters use `name : type`, optionally prefixed by BYVAL. BYVAL is the
+default; BYREF is forbidden for functions (guide section 8.3).
+Array parameters accept `ARRAY OF type` (one dimension) or
+`ARRAY[lower:upper, ...] OF type` (integer literal bounds).
+BYVAL arrays are copied. RETURNS, RETURN, BYVAL, and ENDFUNCTION belong here.
+
+# EXAMPLE
+
+```text
+FUNCTION Double(Value : INTEGER) RETURNS INTEGER
+    RETURN Value * 2
+ENDFUNCTION
+OUTPUT Double(4)
+```
+
+# SEE ALSO
+
+[cpsc-language-overview(7)](cpsc-language-overview.7.md), [cpsc-keyword-procedure(7)](cpsc-keyword-procedure.7.md), [cpsc-keyword-call(7)](cpsc-keyword-call.7.md).
+